This invention concerns improvements in or relating to the control of variable stator vanes in a gas turbine engine, and in particular concerns an improved manner of attaching an actuating lever to a variable stator vane.
During operation of a gas turbine engine using a multi-stage axial compressor the turbine rotor is turned at high speed by the turbine so that air is continuously induced into the compressor, accelerated by the rotating blades and swept rearwards onto an adjacent row of stator vanes. Each rotor blades/stator vanes stage increases the pressure of the air passing through, and at the final stage of a multistage compressor the air pressure may be 30 times ambient atmospheric pressure, i.e. an overall pressure ratio of 30:1.
In addition to translating the kinetic energy of the air into pressure the stator vanes also serve to correct the deflection given to the air by the rotor blades and to present the air at the correct angle to the next stage of rotor blades.
The more the overall pressure ratio of a compressor is increased the more difficult it becomes to ensure that it will operate efficiently over the speed range of the engine. This is because the requirement for the ratio of inlet area to exit area, in the high speed case, results in an inlet area that becomes progressively too large relative to the exit area as compressor speed and hence pressure ratio is reduced. The axial velocity of the air in the front stages thus becomes low relative to the blade speed, changing the incidence of the air onto the blades, and bringing about a condition where the flow over the blades separates and compressor flow breaks down, thereby resulting in a sudden loss of power known as "surging".
Where high pressure ratios are required from a single compressor the above problem is preferably overcome by introducing variable stator vanes into the system. Variable stator vanes permit the incidence of air onto the rotor blades to be corrected to angles which the rotor blades can tolerate.
The manner of operation of a known variable stator vane system will be described with reference to FIG. 1 of the accompanying drawings which shows a cut-away side view of part of a compressor section of an aircraft gas turbine engine.
In FIG. 1 there is shown the compressor section 10 of an aircraft gas turbine engine. In the tubular casing 12 of the compressor section are mounted sets of stator vanes 14 circumferentially about the central axis of the compressor section. A corresponding set of rotor vanes 16 is mounted downstream of each set of stator vanes 14. Each stator vane 14 terminates at the casing 12 in a stem 18 rotatable in a bush bearing 20 on the outside of the casing, the end of the stem extending beyond the bush.
Located externally of the casing 12 and adjacent each set of stator vanes 14 are actuator rings 22 extending circumferentially round the casing. With each stator vane 14 in a set, the vane stem 18 is connected to the corresponding actuator ring 22 by means of an actuating lever 24. One end of the actuating lever 24 is clamped to the end of the vane stem 18 by means of a bolt 26 so that there is no relative movement between the stem and the lever. The other end of the lever 24 is connected to the actuator ring 22 by a pin 28 rotatable in a bush bearing located in the ring.
The actuator ring 22 is arranged so that it may be rotated about the central axis of the compressor section, i.e. in either direction of arrow 9. Consequently, rotation of the actuator ring 22 will, by means of the actuating levers 24, cause rotation of each stator vane 14 about its own axis and thus enable the vanes 14 to assume required angles of incidence to the incoming air. A disadvantage of the geometry of this arrangement is that, as the actuator ring 22 rotates, there is a tendency for each actuating lever 24 to rotate about its longitudinal axis and, because of its stiffness, to loosen the retaining bolt 26. It is further necessary for the actuating lever 24 to resist surge loads in the engine. Hitherto, these problems have been solved by providing forged vane levers 24. However, these are costly, need extensive machining, and generate a weight penalty.
It is an object of the present invention to provide a variable vane actuating lever, and a manner of mounting the lever on a vane stem, such as to overcome the above-mentioned disadvantages.